The invention relates to the general field of telecommunications. The invention relates in particular to a co-operative radio network.
In a co-operative radio network, a plurality of access points co-operate to transmit data simultaneously to a target over a wireless connection. The techniques used, known as co-operative beamforming, enable the signal to interference-and-noise ratio (SINR) to be increased at the target, and thus make it possible to increase the transmission data rate.
Typically, data transmission comprises a plurality of stages:                a relay stage in which source access points having data for transmission to the target transmit the data to other access points of the network, with the data being stored therein;        a training stage, in which the target transmits a training sequence; each access point estimates the impulse response of the channel between the target and itself from the sequence it has received as deformed by the channel, and then each access point determines a pre-equalization filter as a function of the estimated impulse response (e.g. the pre-equalization filter may be the time-reversed impulse response); and        a transmission stage, in which each access point finally retransmits the data while pre-equalizing it with its own filter as determined during the training stage, and in such a manner that the transmissions take place simultaneously and synchronously.        
The advantage of such a technique is that it makes it possible to take advantage of the transmission power of a plurality of access points instead of only one. Let Pmax be the maximum transmission power of an access point. Simultaneous transmission from N access points makes it possible to increase the transmission power by a factor of N compared with Pmax, and in association with the pre-equalization filters, that makes it possible to increase the power received by the target and to increase the SINR. The document entitled “Co-operative time-reversal communication in wireless sensor networks”, by R. J. Barton, Ji Chen, Kyle Huant, Dagang Wu, published in Statistical Signal Processing, 2005 IEEE/SP 13th Workshop on Digital Object, pp. 1146 to 1151 (published in 2005), describes an example of communication in a co-operative network of that type.
The SINR at the target, and thus also the transmission data rate, increase with increasing number of co-operating access points. Nevertheless, the number of co-operating access points is limited by the transmission power of the target Ptarget. Only access points within range of the target can perform the training stage. Unfortunately, the range of the target is limited by its transmission power Ptarget.